The invention relates to an improved liquid cooled mold for the continuous casting of metal and an improved means for securing the liners in the mold.
Liquid metal is continuously cast through a water-cooled mold. The mold consists of liner plates, usually copper, which come into direct contact with the liquid metal being cast. The liner plates usually include cooling water passages and are fastened to a water cooled framework.
The water circulates from the framework through the cooling water passages in the liner plates and back out through the framework, then to heat exchangers and pumps for cooling and recirculation. In this way, heat is removed from the liquid metal being cast. Thus, the liquid metal in contact with the water cooled liner plates solidifies and forms a thin shell. As the cast shape slides along the liner plates, the heat transfer process continues and the solidified shell becomes thicker. In order to have a successful cast, the solidified shell must have sufficient thickness throughout its entire perimeter to support the internal liquid ferrostatic pressure at the point where the cast shape emerges from the mold. This process requires a delicate balance of uniform heat transfer, proper casting speed, selection of correct materials, adequate mold lubrication and good mold design.
The mold liner plates are subject to very severe thermal gradients. The surface temperature of the liner plate in contact with the cast shape varies with location and time. These conditions create extreme stresses in the liner plates which, depending upon design, can cause distortion.
Distortion of the liner plates is detrimental to good uniform heat transfer and, thus, jeopardizes the possibility of successful casting. The liner plates must be machined more frequently, their useful life is shortened and the chances of possible breakout and casting failure are increased. Distortion of the liner plates also causes the molds to leak cooling water. This leakage affects proper heat transfer and, in some cases, creates the possibility of an explosion.
A common method of securing the liner plates to the mold frame consists of drilling and tapping a plurality of blind holes in the liner plate. High strength, corrosion resistant bolts extend through the mold frame and engage the tapped holes in the liner plate. In order to improve the heat transfer, the bottom of the blind holes are machined flat and the bolts are bottomed out against the copper, eliminating the possibility of an air space. The diameter of the bolts is restricted by the spacing of the cooling water passages. The threaded holes in the liner plate are the weak point in the design. The liner plate is subjected to elevated temperatures and the strength of the material is decreased. Copper is the most commonly used liner plate material.
Thermal shock, repeated cycling, and plastic flow are factors which can finally cause the threaded portions of the liner plate to fail.
A method has been used to increase the strength of the connection between the liner plate and bolts. It consists of drilling and tapping oversize holes in the liner plate and then engaging high strength corrosion resistant inserts. The inserts have external threads for engagement in the liner plate and internal threads for engagement with the holding bolts. In this manner, the strength of the connection is increased by increasing the shear area in the threaded portion of the liner plate. This design still has the condition that the heat transfer at the bolts and inserts is different than elsewhere on the liner plate. Also, the use of inserts increases the cost of the liner plates.
Patents which illustrate continuous casting mold of the prior art are U.S. Pat. No. 3,709,286 which teaches the welding of studs to stainless steel strips; and U.S. Pat. No. 3,866,664 teaches the forming of the mold liner with vertical ribs which define with the backup plate a plurality of water passages. Some of the ribs have lateral lips under which metal strips with welded studs are inserted. The studs carried by the metal strips extend through the backup plates for locking the backup plate to the liner. U.S. Pat. No. 3,618,658 teaches utilizing a one-piece or four-piece liner arrangement which is secured to the mold by bolts; U.S. Pat. No. 3,612,158 teaches the forming of the mold wall with inserts of copper which increase the heat conductivity of the mold; U.S. Pat. No. 3,125,786 teaches the utilization of bolts to secure side plates to backing plates to permit longitudinal sliding movement of the side plates relative to the backing plates.